We have reported: 1) A standard method for conducting the mouse bioassay for the detection of estrogenic activity in rodent diets; 2) that rodent diets significantly differ in estrogenic activity; 3) that some rodent diets contain high concentrations of daidzein and genistein equivalent to 4.3 ppb of DES activity and that these phytoestrogens may alter the results of comparative estrogenic and carcinogenicity studies; 4) that metabolizable energy in rodent diets can significantly influence the uterine:body weight ratios of immature CD-1 mice used in uterotrophic assays; 5) that natural ingredient diets can be formulated to contain less than detectable levels of the phytoestrogens daidzein and genistein by omitting the soybean and alfalfa meals; and 6) the dietary phytoestrogens daidzein and genistein significantly increase vaginal openings in CD-1 mice.